Add Metrics API

[tustanivsky]

This PR introduces the Metrics API for sentry-native.

Current progress:

• API for creating, enriching and capturing metrics
• Background metrics aggregation
• statsd-encoding for envelope items
• Periodical metrics flushing
• Normalization for envelope items

Related to #953

[supervacuus]

@tustanivsky, do you already understand why the Windows tests time out?

[tustanivsky]

@tustanivsky, do you already understand why the Windows tests time out?

Not really, things work as expected on Windows when running tests locally.

[supervacuus]

@tustanivsky, do you already understand why the Windows tests time out?

Not really, things work as expected on Windows when running tests locally.

I think it happens because a debug assertion failed, and we are stuck in a dialog on the CI runner. Will investigate.

Since it is also stuck in metics_name_sanitize, it might hint at some sanitizer assumptions that also affect the Android build. 🤞

[cleptric]

As we're fundamentally shifting how Metrics will work at Sentry, I suggest putting this PR on hold for now. At least until we know how the next steps look like.

[supervacuus]

As we're fundamentally shifting how Metrics will work at Sentry, I suggest putting this PR on hold for now. At least until we know how the next steps look like,

Thanks for the heads up, @cleptric.

[supervacuus]

@tustanivsky, I would approve this PR in its current state. At least I don't see any blockers for a merge. But given the news in #985 (comment), I will keep merging blocked.

When this gets unblocked, I would appreciate it if @markushi could also give this a quick review so we have the perspective of another metrics implementer.

CC: @kahest

[JoshuaMoelans]

Closing this PR since Metrics have been deprecated.